Where a continuously cast strand or ingot is formed by means of a magnetic field, a circulating motion of molten metal is due to take place in the liquid portion or crater of the ingot under the action of eddy currents of the electromagnetic origin. This circulating motion of molten metal is known to enhance plasticity of the ingot. At the same time, it has been found that in certain cases, the formation of ingots in magnetic field may give rise to a heterogeneous structure of the ingot in the form of alternating bands which resemble in shape the molten crater of the ingot and which have grains of different structure. Thus, it has been proved that the emergence of the undesirable banded structure is directly associated with the intensity of metal circulation in the liquid portion of the ingot. In other words, the probability of obtaining heterogeneous structure will increase with the intensity of circulation in the molten crater of the ingot. Therefore, to prevent the appearance of the banded structure in the process of the ingot solidification, the intensity of metal circulation in the liquid portion of the ingot should be brought down to a minimum level depending on the ingot size, the alloy composition, and on the requirements to meet the ingot structure.
For example, U.S. Pat. No. 3,605,865 describes a continuous casting process in which the flow of molten metal is continuously fed into a mould and then is subjected to the action of the magnetic field created by energizing an inductor of the mould and to the action of the magnetic field of a stabilizing coil of the mould so that a column of molten metal is formed and then cooled with the resultant formation of an ingot. The process in question is performed by means of a continuous casting apparatus which comprises a mould formed by an electromagnetic inductor and a nonmagnetic stabilizing coil in the form of a triangle having its apex oriented inwardly of the mould and arranged coaxially with the inductor in direct proximity to its butt end.
The stabilizing coil is used to diminish the intensity of magnetic field along the side surface of the column of molten metal so as to permit this column to assume the shape close to that of the cast ingot. However, the stabilizing coil forms together with the inductor a two-phase electrodynamic system creating a travelling magnetic field in other words, the two-phase system comprises two conductors, namely, an inductor and a stabilizing coil, each of which carries alternating electric current differing in amplitude and phase. The electromagnetic force, directed counter to the main circuit, acts to maintain the intensity of magnetic field within the range permitting a stable casting process.
However, the stabilizing coil is ineffective to decrease the intensity of metal circulation in the molten crater of the ingot. This makes it extremely difficult to obtain a homogeneous structure of the ingot, especially in the casting of high aluminium alloys. The produced ingots have a pronounced banded structure, which adversely affects the mechanical properties of the cast product.
What is desired is a continuous casting process and apparatus in which by preventing the circulation of molten metal in the ingot crater and improving the structural arrangement of a stabilizing coil it will be possible to obtain ingots of a homogeneous structure.